1. Field of the Invention
The present invention relates to earth drilling tools for drilling foundations and especially to teeth for drilling heads of rotary drilling tool such as earth drills or rotary buckets.
2. Description of the Prior Art
An earth drill comprises generally a tubular structure forming a shaft around which are formed flights for discharging drilled materials in the shape of an endless or Archimede screw, the lower portion of these flights forming a cutting edge, including a substantially radial drilling head and provided with a plurality of receiving members for removably receiving contoured teeth.
These receiving members, or tooth carriers, are generally provided at their front end with a recess into which the retaining base of a tooth is inserted. In a widely used embodiment, the recess in the tool carrier is formed by a longitudinal groove divided into two side portions by a transverse partition positioned centrally. The corresponding retaining bases of the teeth have thus a U shape in order to be received in the groove and to be maintained transversely therein by the partition of the tooth carrier. The tooth is introduced into the groove, said groove being provided with an elastic coating to prevent inadvertant withdrawal of the tooth.
The working portion of the tooth, or working blade, has generally a tapered longitudinal section, the edge angle and the length of the blade being adapted to the hardness of the soil to be drilled and to the rotational speed of the drilling tool.
This type of teeth presents a certain number of drawbacks when hard soils and especially rocky soils have to be drilled.
On one hand, the shape of the working portion of the tooth, or tooth blade, is not suitable for drilling rocky soils: its mechanical resistance is relatively low and its tapered section, that is its portion which is getting more slender towards the cutting area, does not permit to insert therein tungsten carbide tips which are necessary for proper drilling in such conditions. On the other hand, the retaining arrangement of the tooth base does not permit a perfectly secure retention of the tooth due to the lateral vibrations induced during drilling so that the tooth may be withdrawn inadvertantly.
Special teeth have been proposed in order to permit the insertion of tungsten carbide tips therein. In this type of teeth, the tapered portion of the blade is limited to a short chamfer at the cutting edge of the tooth, the thickness of the blade body being substantially the same as that of the tooth base. This type of tooth presents a double drawback: it does not offer a cutting profile suitable for a good penetration and it increases the overhanging weight of the blade, thus promoting tooth judder and escapement of the tooth from the tooth carrier.